Process for the production of pulps from lignin-containing cellulose material



United States Patent, C

ofDelaware N Drawing. Filed Sept. 2, 1959, Ser. No. 837,596

Claims priority, application Austria Sept. 5, 1958 7 Claims. (Cl. 162-23) This" invention relates to-a process for the production of pulps. from lignin-containing cellulose material. by chemical or semi-chemical digestion of such material and has as. its principal object the reduction of the degradation which normally accompanies such processes.

In .conventional chemical digestion processes more or lesssevere degradation ofthe celluloseoccurs, as is evidentby the reduction in the original value for the molecular weight or the average degree of polymerisation (D.P,.) of the cellulose, In general, the molecular weight or average D.P. of the cellulose drops to about one fifth of the orginal value. It has previously been assumed that this degradation is due to too long a cooking treatment, to the air present in the pores of the cellulosic material being digested and/or to mechanical wear and tear on the cellulose.

Many attempts have been made to eliminate or reduce the degradation which occurs during digestion but Without any substantial degree of success. Thus, for example, experimenters have tried to accelerate the penetrationof the cooking liquor intothje comminutcd cellulosic vmaterial, particularly in the case of wood chips, by increasing or decreasing the pressure used;.by the use of changingpressure and'evacuation; by the use ofdiffcrent tempera: tures, as well as by vaporization (see for example Das Papier, 11, .page 14 (1957); Norwegian Patent No. 54,053; Finnish Patent-No; 17,002; and US; Patent No. 2,072,776). 'It has alsoibeen proposed to digest the mate.-

rialwith caustic soda of various concentrations'in an rounded with gas, e.g. carbon dioxide, hydrochloric, acid.

gas or sulfur dioxide, for the.v purpose of dissolving out lignin and similar incrustations and are. then. shaken.

with concentrated sulfuric acid at room temperature and without pressure (German Patent No. 441,392). The

objectlof-these processes was, on the'one hand, to obtain a' betterwirnpregnation of the material and, on the-other hand, to shorten the actual digestionprocess soas to'obtain a greater through-put of the cookers or disintegrators. Inaddition, thedegrading eifect of the digestion was acribed to the-action of the air remaining-in the pore spaces'of the cellulosic material and it'Was-hoped by preimpregnation to eliminate this air. These attempts, as has previously been stated, have not been'outstandingly successful particularly when thecellulosic material is subjected'before'or during-the chemical action of the digest ing agent to'mechanical treatment such as transport of thematerial oragitation'or mechanical comminution dur-' ing the cooking (see Cellulose Chemie, 16, page 4 "ice 2.. the cellulosic material (preimpregnation) nor-the, air or oxygen in the pores of the material, nor the mechanical treatment before or during the digestion is responsible for the main drop in the average D.P.' of thecelliilose: I have found that the primary cause of the degradation of the cellulose is the oxygen content of the gas'space-above the digestion liquor. Specifically, I have foundthat-when wood chips are digested with bisulfite in the usual manner the digestion liquor always becomes dark or even blackon rapid heating regardless of'wh'ether 'thechip's have been previously impregnated or not if the chips corne'into contact with the oxygen-containing igasatmosphervintthe cookeror autoclave. By contrast, when the airprese'nt the cooker or autoclave is replaced by inert gas, such for example as nitrogen or-carbon'dioxide, the cellulosicmaterial can be heated in a few minutes to the dige'stion'te'mpcrature, evenwithout preimpregnationof the material and withoutthe liquor turning black.

I have further found that pro'vided the chips or other lignin containing cellulosic materialare treated With th'e digesting liquor while avoidingall contact of'th'e cellulosic material with oxygen containingga-s, the liquor does not" turn black and'further the degree of'degradation of the cellulose is considerablyreduced. Thus-thechipsor other material may be so treated that throughout the digestion they are wholly immersed'in the "liquor.

Further, if the atmosphere above the digestion liquor, instead of beingpure nitrogen or pure carbon dioxide, contains up to about 2% (the value varies somewhat with the nature of the cellulosic material) of oxygen the same effect, namely avoiding blackening and avoiding severe degradation of the cellulose, is obtained. Increase in the oxygencontentof the inert gas to 5% brings about a slight.v browning of. the material, while an increase to about 10% causes blackening of the cooking liquor and darkening of the cellulosic material.

According to the'present invention, therefore, pulps are produced frOInJligniH-cOntaining cellulosic materials by digesting them with a" cooking liquor while preventing contactof thecellulosic' material with external gas con taining more than 5% of oxygen. The term external gas is used'in this specification to connote gas in contact with the outside of the cellulosic material and/or in contact with the cooking liquor and is used to exclude the gas contained in the pores of the material since I have determined that the presence of oxygen in the pores of the cellulosic material'exercises no significant effect on the darkening of the cellulose, the blackening of the cooking liquor or the degradation of the cellulose.

While steps may be taken as described above to ensure that thewwooduor, other. cellulosic .material remains completely immersed in the cookingliquor throughout the digestion process, it is preferableaccording to the present invention to till the remainder of the digestor with inert gas.

Thesulfite process iscustomarily'used to obtain the whitest cellulose and-to obtain the highest quality of cellulose; Accordingly, the process of th'e present invention is particularly valuable in relation to the sulfite digesting process. It may, however, equally well be used in the alkali process or in-the sulfate process, it being understood that the latter two processes even with the present invention willnot give'such high-quality cellulose as the sulfite process. The invention further includes both the customary sulfite process using bisulfite with free sulfur dioxide andalso the s'o-called"'neutral sulfite" process.

As is usual when employing an acid sulfite process any suitable soluble bisulfite may be used together with free sulfur dioxide, for example bisulfites of the alkali or alkaline earth metal's'or ammonia, eg. sodium, potassium, ammonium, calcium and magnesium bisulfites.

J1 The ratio of total S to combined S0 may be adjusted 'as is usual in the art.

I have'further determined that given the conditions of the present invention the cellulosic material may be disintegrated or comminuted either before or during the digestion process without seriously degrading the cellulose.

rigorously excluded, I find it advisable to exclude oxygen in any process of transferring of the cellulosic material from one piece of apparatus to another. Again, in

.defibering the pulp or otherwise treating the pulp while in contact with the liquor at fairly high temperature,

it is advisable to exclude air or oxygen. Again this may be achieved by ensuring that the cellulosic material is continuously submerged in the liquor or alternatively the cellulosic material may be blanketed with inert gas.

By the process of the present invention good quality cellulose of high D.P. may be obtained. e.g. in the case of processes not using mechanical disintegration, D.P.s of 1200 or above and in the case of processes using me chanical disintegration D.P.s of 900 and above.

Inasmuch as using the process of the present invention, previous comminution of the cellulosic material does not entail any severe drop of the D.P., the invention permits 4 of the use of sawdust as a raw material. Using sawdust it is generally necessary to stir or agitate during the digestion process.

Normally a sulfite digestion process operates at a temperature of about 130-140 C. or in the case of requiring highly purified pulps at a temperature up to 150 C. Higher temperatures are not possible in conventional processes because of the severe degradation and loss of cellulose which occur. Using the process of the present invention it is possible to cook at temperatures up to about 200 C. with consequent reduction in the cooking time. Funthermore, since according to the present invention preimpregnation of the cooking liquors is unnecessary, or at least produces little or no advantage, such preimpregnation may be omitted, again with resultant shortening of the total processing time.

The following examples illustrate the invention but do not limit it in any way. In the examples where grinding is carried out during the digestion a milling autoclave of the type shown in FIGURE 1 on page 5 of the article in Das Papier, vol. 10 (1956), was used. Where in the examples preimpregnation is used the method is that customarily used in the art, namely evacuation of the autoclave containing the cellulosic material followed by sucking in the cooking liquor, the process being repeated as many times as required. The DR of the products was determined by the Kraemer method (Industrial and Engineering Chemistry, vol. 30, page 1200 (1938)) and lignin was determined by the Tappi Table 1 Gas above liquor Oxygen Nitrogen Trial number 56 56a 56b Mean 55a 1550 Mean value value D.P Not measurable 1, 264 1, 252 1, 269 1, 262

Lignin content (in percent) 33. 5 35 34 34. 8 0. 6 0 3. 2 l. 2

Yield (in percent) 62 63 61- 6 50.8 49. 5 52 50. 8

Defibering properties Does not defiber Very Very Very good good good Appearance of material and spent liquor. Black White Fiber structure Destroyed Maintained pH (of spent liquor) 0.7 0.7 0.8 0.7 1.8 1.9 1.8 r 1.8

Table 2 Gas above liquor Nitrogen Carbon dioxide Oxygen Trial number 53 53a 53b Mean 54 54a 54b Mean 52 52a Mean value value Value Llgnin content (in percent). 14. 8 13. 6 10.8 13. 1 11. 3 12. 6 11. 1 11. 6 25. 6 29. 8 27. 7

Yield (in percent) 60 63. 3 62 62 64. 1 (i1. 9 61. 9 62. 7 83 88 85 Defibering properties Very good Very good Pulveruleut Appearance of materlal Yellowish-white Yellowish-white Brown powder Fibre structure Maintained Maintained Destroyed pH (of spent liquor) 1.8 1.3 I 1.8 1.8 1. 6 1. 4 1. 5 1. 5 1. 2 0. s 1.0

method. In all cases whole knot-free wood was used T able.3 in the formof chips Gas above liquor... Incooker space: Oxygen Incooker space: Nitrogen EXAMPLE, ,I .In the pores: Nitrogen Inthe pores: Oxygen.

This example illustrates digestion using different gases Tmlnumber 58 %?3; 57 573 $332.. above the digestionliquor without grinding during di- 1 gestion. D.P Notmeasurable 1, 996 1, 942 I 1, 969

The wood chips were previously impregnated with sodium bisulfite liquor containing 50 grams per litre of p igmn content (1n total S0 and 10 grams per-litre of combined 50;. The percent) 36.5. 35.2 35.6 3.7 10.9 7.3 ratio of chips to liquor was 1:7. After preimpregnation I of the chips in the autoclave and bringing the liquor up Yield p 62-4, 63 57 to' level, the autoclave w-asclosed and the temperature brought as rapidly as possible .to' 150 C. The cooking igg Prop g gg gg Very time was 10 'hours.= Theresults were obtained as shown Table Appearance of Black White material 1. EXAMPLE II 7 Fiber structure. 7 Destroyed Maintained.

Further trials were carried out using the conditions 4 of Example I with the exception that grinding was carried P D I L75 out continuously throughout the digestion process and the cooking time was 5 hours. The results bt i d are These trials demonstrate conclusively that the presence of Shown in Table; 2; oxygen in the pores" exercises'no substantial degrading Comparison'of the results obtained in Examples I and aflion on the cellulose While the' Presence oflyoxygen n 11 using. nitrogen as the inert gas-above the digestion liquor the space'apov? the coQkmg hquor reduces the DR to a shows that simultaneous grinding reduces the DR somevalue at whlch 1t caimot m t P 3 what andalso considerably'reduces the time takenin results i 57 pliable g may be compared w1th the results obtained in trials 55, digestion. 'In all-cases using inert gas the DP. 1s mam- 55a of Table inExample. I The tamed at a commerclany acceptable valueence between the conditions in the two sets of trialsis that in Table 1 preimpregnation is used whilst in Table 3 EXAMPLE III there is no preirnpregnation. It-will be seen that'very short extension of the cooking time of the trials of Table 3 This example'illu'strates the negligible'etfect of 'gas in gives equivalent results .tothoseobtained in Tablel.v This the pores of the wood chips. It compares the digestion y Short increase in Cooking time is more than Offset y under oxygen of 'woodchips containing nitrogen in the the time used for impregnation in Example pores with the digestion under nitrogen of wood chips con- XA 1 taining oxygen inthe pores. Theairin the ores in the former case wasdisplaced by repeated evacuation. in the Thls examp 1e demonstrates that much'hlgher cooknfg autoclaveand filling with nitrogen. In each case the temperatures may be usedzm the PI-esent pr-ocessthan-m g conventional processes. The. conditions are 1dent1cal with f i was the same as Example'l tne'wok' thoseused -in Example -1 (using nitrogen-above the cookmg 10 hours; cookmgr'temperature150 ing liquor) with the exception of the cooking temperature and the ratio'of wood chips' to cooking liquor was 1:7. which; v 00*? C I andthe cookingtime hi h e The chipYWeTe' not Preimpreghatedi withithe Cooking from 1 /2 to 2 hours. The results obtained areas liquor. Ther'esults are as follows: .follows:. I

Table 4 Digestion time in hours 1% 1% 2.

Trial nuniber 74 74a 74b 740 Mean so 809. Mean 73 73a 73b- 73c 73d. Mean 78.. 78a 78a -78i .72 Mean I value value j value value D.P 1,062 1,059 1, 032 1, 033 1.0 16 956 955 955 859 955 920 997 899 626 493 526 '405 653 532 52a Llgnin content (in ercent)..- 21.8 20.7 20.4 19.2 20.5 16.8 14. 5.. ,15;6 iGA' 16 16.3 16.3 16.9 16.4 4 5.2 5 4:9 :fiil

Yield (in percent) 70.5 74 74 73.4 60.3 62.5 6114 62 59 62.5 62.5 58 6028 46:5 45.8 46.5 45.5 48.8 46.6

Defibering properties Fair Very good Very good Already cornminuted Appearance of material White White Yellowish-white Bright yellow Fibre structure Maintained Maintained Maintained Destroyed pH(oispe11tliquor) 1.7 1.8 1.6 1.7 1.7 1.6 1.8 1.7 1.5 1.6 1.7 1.6 1.5 1.6 1.4 1.5 1.6 1.5 1.7 1.5

Comparison of these results with those obtained in taneously subjecting the cellulosic material to mechanical Example III (Table 3), using nitrogen above the liquor, disintegration and to digestion with an acid sulfite cookshows that at 200 C. approximately the same yield is ing liquor without previous impregnation with said liquor, obtained in 1% to 2 hours as is obtained after 10 hours under an atmosphere which consists, in addition to conat 150 C., while the lignin contents of the products do 5 stituents evaporated from the liquor itself, of inert gas not differ greatly. Extension of the cooking time to 2 /2 containing not more than 5% of oxygen, in a closed hours reduces the D1. considerably at this temperature. digest-or and under the autogenous pressure developed in Accordingly, at 200 C. digestion times of 1%to 2 hours the digestor at the digesting temperature. or a period not greatly in excess of 2 hours may be used. 3. A process for the production of pulp from lignin- Trials were further carried out using above the digescontaining cellulosic material, which comprises simultion liquor nitrogen containing 2%, 5% and 10% of taneously subjecting the cellulosic material to mechanical oxygen. The reaction conditions were 200 C., together disintegration and to digestion with an acid sulfite cookwith mechanical treatment throughout the digestion, i.e. ing liquor at a temperature between 150 and 200 C., the conditions were as severe as possible. With 2% of without previous impregnation with said liquor, under an oxygen in the nitrogen gas the color of the product and atmosphere which consists, in addition to constituents of the spent liquor was very similar to the case when evaporated from the liquor itself, of inert gas containing nitrogen alone was used above the reaction liquor. Using not more than 5% of oxygen. 5% of oxygen in the gas, there was a slight browning of 4. A process for the production of pulps from ligninthe material while with 10% of oxygen in the gas the containing cellulosic material, which consists in simulcooking liquor blackened considerably and in addition taneously subjecting the cellulosic material to mechanthe material itself darkened. These trials demonstrate ical disintegration and to digestion with an acid sulfite that at above 5% of oxygen in the inert gas there is a cooking liquor without previous impregnation with said tendency to darken both in the liquor and in the maliquor, under an atmosphere which consists, in addition terial itself. to constituents evaporated from the liquor itself, of inert EXAMPLE V gas containing not more than 2% of oxygen.

5. A process for the production of pulps from lignincontaining cellulosic material, which consists in simultaneously subjecting the cellulosic material to mechanical distintegration and to digestion with an acid sulfite cook-ing liquor without previous impregnation with said liquor, under an atmosphere which consists, in addition to constituents evaporated from the liquor itself, of substantially pure nitrogen.

6. A process for the production of pulps from lignincontaining cellulosic material, which consists in simultaneously subjecting the cellulosic material to mechanical Table 5 disintegration and to digestion with an acid sulfite cook- This example demonstrates that if the chips are wholly submerged in the liquor throughout the digestion process then results are obtained very similar to those using normal digestion conditions but having nitrogen or other inert gas above the liquor. The cooking liquor and other conditions were the same as those in Example 11 with the exception that trials were carried out both with and without simultaneous grinding, and to ensure complete submersion of the chips the ratio of chips to liquor was 1:21. The results obtained were as follows:

ing liquor without previous impregnation with said liquor, With under an atmosphere which consists, in addition to conffi Wlthwt grindmg 40 stituents evaporated from the liquor itself, of carbon dioxide. Trial number M 30 3 4 6 Mean 7. A process for the production of pulps from ligninvalue containing cellulosic material, which consists in simultaneously subjecting the cellulosic material to mechanical 1,186 1,950 2,040 2,053 1,955 2,009 disintegration and to digestion with an acid sulfite cook- Lignm content (in ing liquor containing a bisulfite whose cation is selected pe 10 14 14 from the group consisting of sodium, potassium, ammo- Yie1d(inpercent) 69 70 71 70 70 nium, calcium, and magnesium and which contains free Defibering very Fair Fair Fair Fair sulphur dioxide, thedigestion being carried out without properties good previous impregnahon with said hquor and under an atmosphere which consists, in addition to constituents Appearance of White Yellowish-white evaporated from the liquor itself, of inert gas containing material not more than 5% of oxygen.

Flber structure.--" M ain- Maintained tamed References Cited in the file of this patent UNITED STATES PATENTS I claim:

1. A process for the production of pulps from lignin- 1,587,631 Clark June 8, 1926 containing cellulosic material, which consists in simul- 1,903,962 Dreyfus Apr. 18, 1933 taneously subjecting the cellulosic material to mechanical disintegration and to digestion with an acid sulfite FOREIGN PATENTS- cooking liquor without previous impregnation with said 201,259 rm ny 1 liquor, under an atmosphere which consists, in addition to constituents evaporated from the liquor itself, of inert OTHER REFERENCES gas containing not more than 5% of oxygen. Scheller: The Influence of Elementary and Active Oxy- 2. A process for the production of pulps from ligningen on Cellulose on the Basis of Viscosity Measurements,

containing cellulosic material, which consists in simul- TAPPI Series XXV, pages 551-555, June 1942. 

6. A PROCESS FOR THE PRODUCTION OF PULPS FROM LIGNINCONTAINING CELLULOSIC MATERIAL, WHICH CONSISTS IN SIMULTANEOUSLY SUBJECTING THE CELLULOSIC MATERIAL TO MECHANICAL DISINTEGRATION AND TO DIGESTION WITH AN ACID SULFITE COOKING LIQUOR WITHOUT PREVIOUS IMPREGNATION WITH SAID LIQUOR, UNDER AN ATMOSPHERE WHICH CONSISTS, IN ADDITION TO CONSTITUENTS EVAPORATED FROM THE LIQUOR ITSELF, OF CARBON DIOXIDE. 